The present invention relates to an improved unloading valve for satisfying the requirements of a hydraulic system while minimizing trasient pressure fluctuations.
It is common in earth-moving equipment, such as front-end loaders, backhoes or the like, to have a bucket or a shovel mounted on a tractor to be raised and lowered, tilted, or otherwise moved into the correct attitude by an appropriate mechanism for the work being performed at the moment. Such adjustments of the bucket or shovel are commonly made by hydraulic cylinders supplied with fluid pressure from a suitable pump.
A common mode of operation in earth-working is to move a bucket or shovel into a pile of material. The hydraulic systems for such earth-working applications require a high volume of fluid at low pressure to rapidly move the cylinder piston rods and, therefore, the bucket or shovel to the work. Then, low fluid volume under high pressure must be available to provide the necessary tilting of the bucket or shovel to break a portion of the material loose from the work pile or lift the material in the bucket or shovel.
One of the prior art approaches has been to provide two fixed displacement pumps to supply the required fluid under pressure with the excess being discharged through an unloading valve. This allows for high flow at low pressure and low flow at high pressure for rapidly moving the bucket to the work and then working the bucket against a load without requiring increased power from the engine.
A problem with known unloading valves that utilize a poppet instead of a spool for the control element is that there are substantial pressure fluctuations at the transition when the valve unloads. In a conventional unloading valve including a ball, seat and plunger spool in the pilot section of the valve, the ratio of the seating area of the ball within its seat to the cross-sectional area of the spool determines the unload to reload pressure ratio. The spool area is greater than the ball seat area for the valve to reset at the desired unloading pressure. This results in oscillations which are introduced between unload and reset pressures due to the difference in areas of the ball seat and plunger spool.
The oscillations within known unloading valves are amplified by the normally low negative gain characteristic associated with these valves. If system pressure and valve inlet pressure rise above the unloading valve setting, the plunger spool tends to push the ball open well beyond its steady state position thereby causing a high net force to act on the valve poppet which opens it quickly. The result is that the valve poppet will overshoot since it will continue opening during the time required for the ball to return towards its seat. This creates an effective low negative gain. Unless the valve dynamic response is carefully tailored to a particular application or the valve is heavily damped, oscillation between unload and reset pressures would result.
Thus, there has been a need for an improved unloading valve which requires no appreciable damping to be stable and which can be made to open quite rapidly thereby protecting against transient pressure fluctuations.